1. Field of the Invention
The present invention relates to the amplification of optical light by stimulated emission, and more specifically, to the reduction of signal crosstalk and amplified spontaneous emission to create a low-noise optical amplifier.
2. Description of Related Art
In a general sense, crosstalk and amplified spontaneous emission (ASE) are the two sources of noise in an optical amplifier. ASE originates from the (spontaneous) emission of incoherent light over the (broad) gain bandwidth of the amplifier. This is the random noise contribution of the optical amplifier. Crosstalk can be categorized as noise by a deterministic signal and arises whenever more than one data channel interacts in an optical amplifier. Crosstalk most commonly originates from data-dependent gain fluctuations at high output powers from the Optical amplifier. This can occur for data multiplexed in either the wavelength or the time domain. In the wavelength domain, gain saturation induced by a data channel operating at wavelength .lambda..sub.1 produces a level change in another data channel at wavelength .lambda..sub.2. In the time domain, gain saturation induced by a data channel occupying time slot t.sub.1 produces a level change for data transmitted at a later time t.sub.2 if an individual bit saturates the gain and the temporal separation (t.sub.2 -t.sub.1) is shorter than the gain recovery time for the optical amplifier.
In optical amplifiers, the above-described noise sources present two limitations on the amplifier operating range. At low input signal levels the amplifier random noise contribution, ASE, causes bit errors (signal-spontaneous beat noise) while at large input signal levels, nonlinearities in the gain medium lead to crosstalk between data channels as described above. Much work has been devoted to extending the useful operating range of optical amplifiers. The two most commercially viable optical amplifiers for optical communications applications are erbium doped fiber amplifiers (EDFA) and semiconductor optical amplifiers (SOA). The gain medium in the EDFA has a comparatively long excited-state lifetime or relaxation time (0.1-1 ms). This allows for a larger saturation energy and hence, for high speed data pulses (.about.1 ns), the gain medium is virtually not saturated [Desurvire et al "Gain Saturation Effects in High-Speed, Multichannel Erbium-Doped Fiber Amplifiers at lambda=1.5 .mu.m", J. Lightwave Tech., vol. 7, no. 12, pp. 2095-2104 (1989)]. Time domain crosstalk is not an issue for high speed data in EDFA's. An automatic gain control (AGC) circuit at the detection end can compensate for fluctuations in the data stream that occur on the order of 1 ms. However, significant crosstalk between channels can occur for multichannel applications such as wavelength division multiplexing (WDM), especially when one channel is dropped or added to the system. AGC can be used, but only if the wavelength filter used for demultiplexing is further constrained. An inline and counterpropagating lasing field with an EDFA cooled to 77.degree. K. has been used to inhomogeneously broaden and thereby spectrally separate and equalize the gain curve [da Silva et al "Automatic Gain Flattening in Optical Fiber Amplifiers Via Clamping of Inhomogeneous Gain", IEEE Photonics Tech. Lett., vol. 5, no. 4, pp. 412-414 (1993)]. However, this device has limited usefulness since a refrigeration mechanism is required, it is intrinsically unidirectional in amplification of the signal beam, and the crosstalk is suppressed only for wavelength channel spacings greater than the inhomogenous linedwidth. In order to reduce the system complexity, a more elegant solution to the EDFA (WDM) crosstalk problem is needed.
U.S. Pat. No. 3,467,906 discloses a transverse lasing field used to obtain a gain medium that is independent of pumping current. This patent claims to suppress the ASE along the amplification path. This invention does not suppress the parasitic lasing modes in the highly multimode lasing structure. The parasitic lasing modes often take the form of low loss circulating modes and prematurely clamp the gain at a level that is too low to be useful. These circulating modes often arise from totally internally reflected paths within the structure. In addition, these inventors did not take into account carrier-carrier scattering in stating that the noise output along the length of the amplifier is suppressed by the, transverse lasing field. The amplifier noise in this structure is suppressed only with a commensurate suppression of gain at that wavelength. In other words, the stated structure will not improve the output signal-to-noise ratio over a conventional optical amplifier.
Crosstalk is considered to be the most difficult problem to solve in SOAs. Some researchers have identified crosstalk as the problem preventing widespread use of SOA's [Brierly et al "Progress on Optical Amplifiers for 1.3 .mu.m", Tech. Digest. Optical Fiber Communications Conference, paper TuL3, p. 67 (1992)]. Crosstalk severly limits the operation of SOAs for both high speed data and multichannel applications such as WDM [Jopson et al "Measurement of Carrier-Density Mediated Intermodulation Distortion in an Optical Amplifier", Electronics Lett., vol. 23, no. 25, pp. 1394-1395 (1987), Koga et al "The Performance of Traveling-Wave-Type Semiconductor Laser Amplifier as a Booster in Multiwavelength Simultaneous Amplification", J. Lightwave Tech., vol. 8, no. 1, pp. 105-112 (1990), Saleh et al "Effects of Semiconductor-Optical-Amplifier Nonlinearity on the Performance of High-Speed Intensity-Modulation Lightwave Systems", IEEE Trans. on Communications, vol. 38, no. 6, pp. 839-846 (1990)].
All these crosstalk mechanisms originate from data-dependent gain fluctuations at higher output powers from the SOA. The carrier lifetime is comparatively short in the most common semiconductors used for laser diodes and optical amplifiers (.about.1 ns). This time constant is presently not attainable with AGC circuits. Thus, data that has fluctuations on the order of a 1 Gb/s and output powers levels on the order of the saturation power have severly degraded detection fidelity [Saleh et al "Effects of Semiconductor-Optical-Amplifier Nonlinearity on the Performance of High-Speed Intensity-Modulation Lightwave Systems", IEEE Trans. on Communications, vol. 38, no. 6, pp. 839-846 (1990)]. For example, a typical double heterostructure amplifier has an output saturation power of 3 dBm and a gain of approximately 25 dB. The input power must be less than -22 dBm in order to avoid signal crosstalk in the time domain (intersymbol interference). Similar constraints apply to WDM crosstalk. Researchers have tried to solve the crosstalk problem using feedforward methods [Saleh et al "Compensation of Nonlinearity in Semiconductor Optical Amplifiers", Electronics Lett., vol. 24, no. 15, pp. 950-952 (1988), Nyairo et al "Multiple Channel Signal Generation Using Multichannel Grating Cavity Laser with Crosstalk Compensation", Electronics Letto, vol. 28, no. 3, pp. 261-263 (1992)]; however, these schemes are limited in operating range, and do not solve the general problem of crosstalk for multichannel applications.
Other researchers have tried to solve the problem in the time domain by decreasing the carrier lifetime [Eisenstein et al "Gain Recovery Time of Traveling-Wave Semiconductor Amplifiers", Appl. Phys. Lett., 54 pp. 454-456 (1989)]. This type of solution does not improve crosstalk performance for wavelength multichannel applications. Another direct approach is to increase the output saturation power of the amplifier by using quantum wells as the gain medium. As described in [Tiemeijer et al "Polarization insensitive multiple quantum well laser amplifiers for the 1300 nm window", Appl. Phys. Lett., 62 pp. 826-828 (1993)], a high saturation power, polarization independent, low coupling loss device has been developed using low confinement factor quantum well devices to increase the saturation power. Even though the saturation power is increased by approximately 10 dB, the crosstalk problem is still present and only shifted up in power by 10 dB.
Another approach to increase the output saturation power is to use a tapered amplifier design [Koyama et al "Multiple-Quantum-Well GainAs/GainAsP Tapered Broad-Area Amplifiers with Monolithically Integrated Waveguide Lens for High Power Applications", IEEE Photonics Tech. Lett., vol. 5, no. 8,. pp. 916-919 (1993)]. This particular design requires almost an order of magnitude more pump current than required by conventional SOAs. One could think of increasing the carrier lifetime to the order of a microsecond in a n-i-p-i structure thus approaching the crosstalk situation of an EDFA; however, the gain length would be increased by several orders of magnitude which is unreasonable for a semiconductor device. For optical amplifiers in general, a device inherent solution to the crosstalk problem is needed. One researcher has used an on-axis lasing field to speed the modulation response of an SOA [U.S. Pat. No. 5,119,039 ] but no mention of crosstalk reduction was made in either the time or spectral domains.
U.S. Pat. No. 5,184,247 discloses a spectral filter for use in a waveguide structure with gain in order to stabilize the gain medium by lasing the structure at the passband wavelength of the spectral filter. This invention does not allow for practical separation of the resulting laser light from the amplified signal. A separate filter must be used to suppress the laser light in the direction of the amplified signal. In a specific embodiment where a transverse lasing field was employed in conjunction with a perpendicular waveguide, the inventor includes a saturable absorber section which only allows it to be used as a nonlinear wavelength conversion device or pulse regenerator, as the inventor points out. This particular embodiment does not mention a method of suppressing the circulating and/or other pararsitic lasing modes of this structure. Futhermore, no method for suppressing the ASE in this type of optical amplifier is presented.
An improvement in the signal to random noise ratio for an optical amplifier is useful only if the ASE power can be decreased without sacrificing the signal gain. The traditional method is to use spectral filtering since the spectrum of the ASE is much broader than that of the signal [Olsson "Lightwave Systems with Optical Amplifiers", J. Lightwave Tech., vol. 7, no. 7, pp. 1071-1082 (1989)]. The filter bandwidth cannot be arbitrarily narrowed, however, because systems requirements will not tolerate the need to accurately match and stabilize the source wavelength and central wavelength of the filter. Thus, even after spectral filtering the signal-spontaneous beat noise dominates. An alternative method for decreasing the ASF power without sacrificing signal power is desirable. Some researchers have used an aperture to spatially filter the ASE [Kogelnik et al "Considerations of Noise and Schemes for Its Reduction in Laser Amplifiers", Proc. IEEE, vol. 52, pp. 165-172 (1964); EP 0430911 A1; U.S. Pat. No. 4,551,684] in optical amplifiers, but the exponential gain of the ASE is not altered. One researcher has patented an off-axis laser cavity to decrease ASE noise [JP 2246181 A]; however, the effectiveness of this invention is not realized since it reduces ASE emission in the amplified direction at the expense of decreasing the gain overlap and hence; the gain of the amplified signal. This solution does not improve the output signal-to-noise ratio of the optical amplifier as implied.